The present invention relates to a light scanning unit for use in, for example, a laser printer, a digital copier and the like and, particularly, to a light scanning unit of an over-illumination type in which a width in a main scanning direction of a luminous flux made incident on a polygon mirror (a direction along a rotary direction of the polygon mirror) is greater than a width in the main scanning direction, of reflective surfaces of the polygon mirror.
As the light scanning units of the over-illumination type invented by the inventor, there are Jpn. Pat. Appln. KOKAI Publication No. 2002-328323 and U.S. patent Ser. No. 10/131,207 (filed Apr. 25, 2002) corresponding thereto.
The width in the main scanning direction of a light beam deflected by the polygon mirror is constant irrespective of a scanning angle (position angle) in a light scanning unit of under-illumination type though it is varied in accordance with a scanning angle in the above-mentioned light scanning unit of the over-illumination type.
In the present inventor's senior application, too, however, the problem that if the light beam made incident on the polygon mirror forms an angle with an optical axis of an image-forming optical system on a main scanning plane (if the light beam is made incident obliquely), irregularity in a cross-sectional beam diameter is laterally asymmetric about a center of the optical axis of the image-forming optical system, in the main scanning direction, is not solved completely.
When the cross-sectional beam diameter is irregular and laterally asymmetric about the center of the optical axis of the image-forming optical system, there is a problem that a latent image formed on a photosensitive body, i.e. an image density is varied at each scanning position.
Incidentally, if the light beam is made incident on an arbitrary reflective surface of the polygon mirror from a front face in the main scanning direction (in the vicinity) to reduce the irregularity in the cross-sectional beam diameter of the light beam on an image face (photosensitive body), the light beam traveling between a lens group, which is provided between the polygon mirror and the photosensitive body, and the arbitrary reflective surface of the polygon mirror forms an image within an image region of the photosensitive body and thereby causes degradation in image quality. The image quality is improved by the above proposal of the present inventor, but the degradation is not solved completely.
Even if a reflection prevention film is provided on the lens surface of the arbitrary lens to reduce the irregularity in the cross-sectional beam diameter of the light beam, the manufacturing costs of the lens are increased, profile irregularity of the lens surface is deteriorated and the optical characteristics are worsened as already disclosed by the present inventor.